From: cauldwell.thomas@nospicedham.gmail.com   
      
   On Monday 17 July 2023, Frederick Gotham wrote:   
   >   
   > If I change it to thread_local then try to re-compile, I get a linker error:    
   >    
   > R_AARCH64_ADR_PREL_LO21 used with TLS symbol f    
   >    
   > Do you know what syntax I use to access the thread_local variable from   
   assembler? Will I need to write a separate function as follows?    
      
      
   In order to try understand how thread_local variables are accessed from   
   aarch64 assembler, I wrote the following dynamic shared library in C:   
      
         __thread void (*f)(void);   
      
         void (*g)(void);   
      
         void Func(void)   
         {   
             g = f;   
         }   
      
   I compiled this to 'libtest.so" and then used 'objdump' on it to see:   
      
                     :   
   Line 01:       stp     x29, x30, [sp, #-16]!   
   Line 02:       mrs     x1, tpidr_el0   
   Line 03:       mov     x29, sp   
   Line 04:       adrp    x0, 20000 <__cxa_finalize>   
   Line 05:       ldr     x2, [x0, #16]   
   Line 06:       add     x0, x0, #0x10   
   Line 07:       blr     x2   
   Line 08:       adrp    x2, 1f000 <__FRAME_END__+0x1e8c8>   
   Line 09:       ldr     x2, [x2, #4032]   
   Line 10:       ldr     x0, [x1, x0]   
   Line 11:       str     x0, [x2]   
   Line 12:      ldp     x29, x30, [sp], #16   
   Line 13:      ret   
      
   Line #2 appears to put the address of "thread local storage" inside the x1   
   register.   
   Lines #4-7 at first glance seem to call the function "__cxz_finalize" (which   
   is the one that gets called at the end of a program to invoke all the   
   destructors of global objects)... but really I just think that the number   
   0x20000 is being used as a base    
   address to apply offsets to.   
   Lines #7 definitely is calling some function though, I don't know which one.   
   Lines #8-12, I'm not sure here... but I think they're moving the value of the   
   thread_local variable 'f' into the global variable 'g'.   
      
   Can anyone please help me understand this? And explore how I would go about   
   writing aarch64 to access a thread_local variable called 'f'?   
      
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